1. Field of the Invention
The present disclosure relates to methods and apparatus for treating articles during their formation. Certain embodiments described within the present disclosure more particularly relate to methods and apparatus useful for limiting segregation in and/or for improving the surface quality of an article forming from a molten metal or alloy, as well as to alloys and articles of manufacture made using the methods of the present disclosure.
2. Description of the Invention Background
During alloy production, a consumable electrode may be melted and refined by methods such as vacuum arc remelting (VAR), electroslag remelting (ESR) and electron beam melting (EBM) to form an ingot. During the melting and refining process, a portion of the alloy typically is in the form of a molten pool on the surface of the forming ingot. The molten pool progressively cools and solidifies, forming the ingot. Frequently with VAR, ESR, or EBM, a stationary water-cooled copper mold is used to form the ingot. The mold facilitates the cooling process by transferring heat from the ingot. The typical cooling rate of an ingot formed using a water-cooled mold, however, is relatively slow. The cooling rate of an ingot is dependent on location within the mold and can be represented by a factor termed “local solidification time” (LST). LST is defined as the time taken for the center of an ingot to solidify from the initial molten state. As an example, LST can be approximately 3000 seconds in a conventional cylindrical 20-inch (about 50.8 cm) diameter ingot of Alloy 718 formed by VAR.
Slow cooling during ingot solidification may result in unacceptable alloy segregation. Segregation is a casting defect involving a regional concentration of alloying elements and is commonly expressed as the departure in a particular region of a cast article from an average chemical composition of the article. Segregation can have deleterious effects on an alloy's mechanical properties, such as, for example, tensile strength, elongation, hardness, fatigue limit, and corrosion resistance. Melt-related segregation can take the form of “freckles”, which are manifested as dark spots on a macroetched specimen of solidified alloy. Segregation is difficult to remedy by thermomechanical treatment.
VAR, ESR and EBM apparatus typically incorporate “withdrawal” molds. A withdrawal mold commonly includes a translatable base that supports the ingot and moves progressively away from the melting device as the electrode is melted. The ingot surfaces form within a hollow mold that is at a generally fixed position relative to the melting portion of the electrode.
Continuous movement of an ingot through a withdrawal mold of a VAR, ESR, or EBM apparatus can produce surface defects on the ingot. As the ingot forms, the molten alloy pool on the top surface of the ingot forms a meniscus at its interface with the hollow mold. As the molten pool solidifies, the ingot may adhere to the hollow mold. Although not intending to be bound by any theory, it is believed that this adhesion is mechanical in nature. As the mold base of a withdrawal mold translates away from the melting device, the ingot is progressively withdrawn from the hollow mold, and the newly solidified alloy surface is broken away from the mold wall. This may produce surface defects, which can take the form of, for example, tears and cracks on the surface of the ingot. (Analogous surface marks formed during continuous casting are commonly referred to as “witness marks” and are caused by the motion of the mold relative to the casting.) It may be necessary to remove surface defects from an ingot before further processing. Surface defect removal reduces yield, adversely impacting economics, and also lengthens the time necessary to produce a finished ingot.
Accordingly, it would be advantageous to provide a method and apparatus for forming ingots of alloys provided by ESR, VAR, EBM and other melting and/or refining techniques wherein the ingots exhibit reduced segregation and/or surface defects relative to ingots of like composition provided by conventional methods. More generally, it would be advantageous to provide a novel method and apparatus for treating articles during their formation from molten material, wherein the quality of the formed article is improved in one or more respects relative to articles formed by conventional methods and apparatus.